1. Field of the Invention
This invention relates generally to a multiple speed step-ratio automatic transmission for a motor vehicle and, in particular, to the control of a coasting downshift performed by the transmission.
2. Description of the Prior Art
During a coasting downshift of an automatic transmission in which shifts occur between individual gear ratios, a lash transition can occur when torque converter turbine speed and engine speed (or torque converter impeller speed) cross at too high of a rate. This lash transition excites components in the transmission and driveline system, which can result in an audible clunk disturbance to the vehicle operator and passengers. Under certain conditions, the lash can cross and cross back, and the customer may experience both an excessively long gear shift and a double clunk.
The technical reason for the clunk during the coasting downshift sequence is the rate at which the on-coming synchronous element, such as a brake comprising a band and drum, gains torque capacity relative to the off-going synchronous element losing torque capacity. The relative element capacities are determined by estimated engine torque, engine inertia, and vehicle deceleration rate, which are translated into hydraulic control pressures and apply forces acting on the elements. This translation is further impacted by a rise in the interface temperature of the on-coming band and drum, which increases the likelihood of a torque reversal and subsequent lash transition during the gear shift.
The apply force on the band is proportional to torque for both an on-coming friction control element and an off-going control element. The torque reversal causes a crossing of driveline lash, which creates a torque disturbance and the audible clunk.
A need exists in the automotive industry for a technique to adjust the relative pressures of the on-coming and off-going control elements during a coasting downshift to reduce lash and therefore minimize the rate at which that lash is crossed based on system level inputs.